Field of the Invention
The invention concerns a gradient coil arrangement for a magnetic resonance apparatus, composed of a number of sub-coils formed by coil conductors on a common substrate. The invention also concerns a magnetic resonance apparatus and a method for damping such a gradient coil arrangement in the scanner of a magnetic resonance apparatus.
Description of the Prior Art
Gradient coil arrangements of the above-described type are typically used in magnetic resonance apparatus, in the magnetic resonance scanner thereof, for encoding the magnetic resonance signals in different spatial directions. For this purpose, a number of sub-coils are disposed on a common substrate and sealed, e.g. in encapsulating material. The sub-coils are formed by a coil conductor, e.g. a copper wire of circular or oblong cross section, and/or implemented as conductor tracks. The sub-coils are supplied from a voltage source individually or as an interconnected group. The signals applied can be both DC voltages and voltages having other spectral components. Switching signals of a power amplifier having a clocked switching output stage may be present on one or more sub-coils. Moreover, these switching signals have a typical switching frequency on the order of 20 to 50 kHz, and exhibit corresponding harmonics that are technically relevant up to the 1 MHz range.
Because of the numerous sub-coils that form a primary and/or secondary layer of the gradient coils, and possibly a number of shim coils for adjusting the homogeneity of a B0 field of the magnetic resonance scanner, and the spatially tight arrangement of their coil conductors, there is strong inductive and capacitive coupling between the sub-coils. This, together with the geometrically complex arrangement of the coil conductors, produces a resonance behavior involving natural frequencies that may coincide with the spectrum of the switching signal. If these natural frequencies are excited by the switching signal, locally large currents may be produced that heat the coil conductor unacceptably in this region. Additionally, the resulting electrical fields may exceed the breakdown field strength of the substrate material or inclusions present therein, thus producing spontaneous discharges. If a discharge or partial discharge has a power density in the reception band of the magnetic resonance apparatus, the discharge or partial discharge can severely impair imaging in the form of an artifact known as a spike. In extreme cases, even the insulation between the sub-coils can be damaged by such discharges.
To reduce these undesirable effects, it has been proposed to provide low-pass filters at the output of the switching output stage in order to suppress unwanted spectral components. A disadvantage of this approach is that such a low-pass filter also produces undesirable attenuation and phase shifts in its pass band. Moreover, such a filter is very complex/costly to implement because it must be designed to handle high currents and voltages (up to 1 kA and 2 kV respectively).